1. Field of the Invention
My invention relates to bin level sensors for sensing the presence of particulate material, such as comminuted, granular, or pulverous material, at a particular level in a bin or hopper, and more particularly to photoelectric bin level sensors.
2. Description of the Prior Art
(It is to be understood that the term "prior art" as used herein or in any statement made by or on behalf of applicant means only that any document or thing referred to as prior art bears, directly or inferentially, a date which is earlier than the effective date hereof.)
Photoelectric bin level sensors are known in the prior art. For example, a photoelectric bin level sensor is shown and described in U.S. Pat. No. 4,100,538, which was issued to John T. Knepler on July 11, 1978. A copy of this prior art patent is submitted herewith.
Photoelectric level sensors for liquids and sludges in tanks are also known in the prior art, as seen in U.S. Pat. Nos. 1,958,252; 3,441,737; 3,644,914; and 3,683,347.
Electromechanical bin level sensors for particulate materials in bins and hoppers, such as those which sense the presence of particulate material by means of the resistance of that material to the rotation of a motor-rotated paddle or vane, are also well known in the prior art. Such a rotating paddle bin level sensor is shown and described in U.S. Pat. No. 4,147,906, which was issued to Walter E. Levine on Apr. 3, 1979.
Other types of bin level sensor, based upon different operating principles, such as diaphragm displacement, electrical capacitance, ultrasound, low frequency vibration, and radio frequency electromagnetic radiation, are also known in the prior art.
With the exception of the diaphragm displacement devices, which have their own problems, the prior art devices in general comprise relatively long or bulky probes, which protrude into the bin or hopper and contact the particulate material stored therein. These elongated or bulky probes are likely to be damaged by the weight of the particulate material in the bin or hopper.
The rotating paddle type bin level sensors of the prior art are relatively costly, being typically priced at $120 to $200 each, and are generally unreliable, in that in most applications such a sensor will last no more than a few months at a time without repair.
The prior art devices described above which have no moving parts suffer in general from other problems, such as high cost and insensitivity, i.e., too small an output signal difference between the presence of material in the bin and the absence of material from the bin, which causes an unreliable signal as operating conditions change. Additionally, the bin level sensors of this class generally employ bulky or elongated sensing probes which are subject to damage by the weight of the material in the bin or hopper.